Concerning power saving control (sleep control) in a PON (Passive Optical Network) system in the past, efficiency and a method for the power saving control have been discussed. For example, an OLT (Optical Line Terminal) transmits a control message to an ONU (Optical Network Unit) to set the ONU in a power saving state (a sleep mode) for a designated time. For example, Patent Literature 1 examines a method of performing automatic switching concerning whether both transmission and reception are set in the sleep mode or transmission is set in the sleep mode in the sleep control.
On the other hand, a system operator sometimes performs failure monitoring for a link failure or the like to perform communication management. In performing the failure monitoring, in some case, the system operator manages a power supply state of the ONU to perform the communication management. Consequently, in some case, the system operator performs, for example, isolation of a communication path failure.